1. Field of the Invention
The present invention relates to a superconducting electric rotary machine. More particularly, it relates to a rotor for a superconducting electric machine having a cooling structure for maintaining a superconducting field coil in the rotor under a superconducting transition temperature.
2. Description of Prior Art
As a rotor for a superconducting electric rotary machine of this kind, there has been proposed such a one as shown in Japanese Unexamined Patent Publication No. 22372/1982.
FIG. 27 is a longitudinal cross-sectional view of such rotor. In FIG. 27, reference numerals designate a torque tube 1, a coil supporting tube 2 constituting the intermediate portion of the torque tube 1, a superconducting field coil 3 mounted on the coil supporting tube, an ambient temperature damper 4 (hereinbelow, referred to as first damper 4) which surrounds the torque tube 1 and the coil supporting tube 2, a radiation shielding damper 5 (hereinbelow, referred to as second damper 5) which is disposed between the first damper 4 and the coil supporting tube 2, an outer tube 6 attached to the outer periphery of the coil supporting tube 2 to cover the superconducting field coil 3, a pair of end plates 7 attached to both sides of the coil supporting tube 2, respectively driven and driving end shafts 8 and 9 extending from both sides of the rotor, bearings 10 for supporting the end shafts 8, 9, slip rings 11 for feeding a field current, a heat exchanger 12 formed in one piece with or attached to the torque tube 1, a pair of side plates 13 for shielding radiation, and vacuumed portions 14.
In the rotor of the superconducting electric rotary machine having the above-mentioned construction, when the superconducting field coil 3 mounted on the coil supporting tube 2 is cooled to a cryogenic temperature, the electric resistance of the coil becomes substantially zero and the excitation loss is eliminated, whereby a strong magnetic field is generated by the superconducting field coil 3 and a.c. power is generated from a stator (not shown). In order to cool the superconducting field coil 3 at a cryogenic temperature, liquid helium is supplied from the center bore of the end shaft 9 (which is at the opposite side of the driven end shaft 8) through a feeding tube (not shown) to a liquid helium vessel which is formed by the coil supporting tube 2, the outer tube 6 and end plates 7; spaces 14 in the rotor are kept under a highly vacuumed condition; the torque tube 1 for transmitting a torque to the coil supporting tube 2 with the superconducting field coil 3 cooled at the cryogenic temperature is formed by a thin cylindrical body and is provided with a heat exchanger 12 or heat exchangers 12, so that heat invading through the torque tube 1 to the cryogenic temperature portion is reduced to a minimum. The side plates 13 are so provided that heat of radiation from the side of the end plate 9 can be reduced.
The first and second dampers 4, 5 are adapted to shield a magnetic field having high harmonics from the stator to protect the superconducting field coil 3 and adapted to attenuate the vibrations of the rotor due to a disturbance in the power system. In addition, the first damper 4 functions as a vacuum outer tube, and the second damper 5 functions as a radiation shield to the helium vessel portion. In FIG. 27, piping for constituting a helium introduction and discharge system for the rotor and a helium introduction and discharge device connected to the rotor are omitted.
FIG. 28 is a cross-sectional view of a part of the torque tube 1 taken along the line VII--VII. In FIG. 28, numerals designate a section for storing the liquid helium 15, a space 16 in which vapor helium is filled, a slot 17 receiving a part of the superconducting field coil 3 mounted on the coil supporting tube 2, an earth insulator 18 between the superconducting field coil 3 and the coil supporting tube 2, a wedge 19 for fixing the coil 3, an opening 20 formed between the coil supporting tube 2 and the outer tube 6, helium passage 21 for communicating the section for storing the liquid helium 15 with the slot 17, and through-holes 22a, 22b in, for instance, circular form which are formed in the earth insulator 18.
As a superconducting field coil used for the rotor having the construction as above-mentioned, there has been proposed such a one as disclosed in, for instance, Japanese Unexamined Patent Publication No. 186960/1982. FIG. 29 shows the superconducting field coil disclosed in the above-mentioned publication.
In FIG. 29, the superconducting field coil 3 is formed by winding a superconducting wire 3a on the coil supporting tube 2 in plural numbers of lines and layers. The superconducting wire 3a is formed by twisting a plurality of electric wire elements. A numeral 23 designates in-line insulators inserted between adjacent rows of the wound superconducting wire 3a and numeral 24 designates layer insulators inserted between vertically adjacent layers of the wound superconducting wire. The insulators 23, 24 are made of an insulating plate-like material, not having any grooves or apertures. The in-line insulators 23 and the layer insulators 24 are inserted between laterally adjacent superconducting wire 3a and vertically adjacent layers of the superconducting wire 3a one by one, while the single superconducting wire is wound. On completion of winding operations, the wound body is treated by an epoxy resin in a molded form, whereby spaces between the adjacent superconducting wire 3a are filled with insulation to prevent a short circuit.
Generally, in the superconducting electric rotary machine, there is a technical problem as to how the superconducting field coil is cooled at a cryogenic temperature. It is necessary to cool the coil below the superconducting transition temperature in order to keep the coil in the superconducting condition. The cooling operation is carried out by using helium as a cooling medium to give a temperature range of from 1 K to 20 K (the absolute temperature scale). However, since the specific heat of the superconducting field coil becomes extremely small in such cryogenic temperature condition, the temperature of the coil increases even by a small amount of heat produced in the coil or by leakage of a small amount of heat into the coil, whereby the temperature of the superconducting field coil may exceed the superconducting transition temperature. Accordingly, an important point in design of the superconducting electric rotary machine resides in how the heat produced in the coil itself and the heat invading the coil can be quickly removed.
The heat in the coil 3 or the heat entering into the coil 3 is absorbed by the helium which fills small gaps between the superconducting field coil 3 and the earth insulators 18 surrounding the coil 3. When the helium absorbs the heat, volume expansion is caused. Thus, the helium, having a small density, passes through the holes 22a formed in the earth insulator 18 due to natural convection in a centrifugal force field and then, it flows into the helium liquid storing section 15 through the helium passage 21 in the coil supporting tube 2. On the other hand, shortage of helium is caused in spaces around the superconducting field coil 3. Accordingly, the area around the superconducting field coil 3 is supplied with the helium which flows from the opening 20 communicated with the inner space of the outer tube 6 through gaps in the wedge 19 and the through hole 22b of the earth insulator 18. The helium which reaches the liquid storing section 15 has a lesser density and is subjected to partial evaporation and cooling. The cooled helium is circulated through a course of another helium passage 21, another through-hole 22a in the earth insulator 18, the area around the coil 3, another through hole 22b in the earth insulator 18, gaps in the wedge 19 and the opening 20.
Thus, the cooling of the superconducting field coil 3 is carried out by natural circulation as above-mentioned so that it is kept at a temperature lower than the superconducting transition temperature.
However, the conventional superconducting electric rotary machine had the problem as described below. Namely, since the cooling of the superconducting field coil 3 is carried out only from the side of its outer circumferential surface, when heat is produced in the superconducting wire 3a constituting the coil 3, the heat in the superconducting wire 3a has to be removed to the outer circumference of the coil 3 through the in-line insulators 23, the layer insulators 24 and the superconducting wire 3a by the cooling function of the helium around the coil 3. Accordingly, the poor cooling effect in the conventional machine causes increase in the temperature of the superconducting wire 3a with the consequence of inviting the breaking (quenching) of the superconducting condition.
There has been known a technique as shown in FIGS. 30 and 31 (which is proposed by, for instance, Japanese Unexamined Patent Publication No. 202852/1982) in order to remove the heat produced in the superconducting field coil and the heat entering it. In FIGS. 30 and 31, the same reference numerals as in FIGS. 27 to 29 designate the same or corresponding parts. In FIGS. 30 and 31, a reference numeral 30a designates a plurality of circular through holes formed in an upper insulating packing, a numeral 31a designates a plurality of circular through holes formed in a lower insulating packing 31 and a numeral 38 designates a side insulating packing provided at the side part of the superconducting field coil 3 in the slot 17. However, in the rotor having the construction as above-mentioned, the through holes 30a of the upper insulating packing 30 and the through holes 31a of the lower insulating packing 31 are respectively formed in the radial direction with respect to the coil supporting tube 2, and the distances between the through holes 30a and between the through holes 31a are relatively large with respect to the axial direction of the coil supporting tube 2. In this case, a pass for transmitting heat produced in the superconducting field coil 3 in which the through holes 30a, 31a are relatively close to a heat generating part is more advantageous than a path in which the through holes 30a, 31a are relatively remote. Accordingly, when a small amount of heat is produced or enters at a position remote from the through holes 30a or 31a, it is difficult for the helium which absorbs the heat to flow through the helium passage in the coil supporting tube 2. Then, the temperature of the coil 3 increases, thereby to cause the quenching.
In the rotor having such construction that the superconducting field coil 3 is held in the slot 17 formed in the coil supporting tube 2 and the coil 3 is kept at the superconducting transition temperature or lower, it is necessary to provide insulators having a sufficient insulating properties so as not to cause the break down of insulation, because a voltage of several hundreds to about 1000 volts takes place if the superconducting condition is lost (there is substantially no voltage in the superconducting field coil 3 under the superconducting condition).
For cooling the superconducting field coil, it is considered that through holes 30a or 31a are formed in the upper and/or lower insulating packing 30, 31 to pass the helium. However, formation of the through holes 30a, 31a reduces insulating properties. In order to prevent the reduction of the insulating properties, the thickness of the upper and/or lower insulating packing 30, 31 is increased to provide a sufficient insulating length. However, the insulating packings occupy a large volume in the slot 17. This involves reduction of the space for the duper ducting field coil 3, with the consequence that the capacity of the superconducting field coil 3 decreases. This is a great problem in the function of the superconducting electric rotary machine. If parts in the slot 17 other than the insulators have the same volume, the depth of the slot 17 has to be large for the increased thickness of the upper and/or the lower insulating packing 30, 31. This makes the conventional machine non-efficient.